As is known, gas lasers have been developed in the past wherein one gas in its metastable vibrational level can be used to selectively populate an upper level of another gas through resonant transfer via inelastic collisions. One such system is described in Patel U.S. Pat. No. 3,411,105 wherein the vibrational energy of nitrogen is transferred to carbon dioxide, the active laser medium. In the carbon dioxide-nitrogen laser, an electric discharge in a mixture of these gases results in collisions of electrons in the discharge with nitrogen molecules, thereby exciting them vibrationally. The cross section for these excitation processes is quite high. Since the nitrogen molecule, like all homonuclear diatomic molecules, possesses no intrinsic electric dipole moment in any of its vibrational states, relaxation of vibrationally excited nitrogen via emission of radiation is impossible. An excited molecule, therefore, retains its excess energy until it gives it up by collision which can be either with a container wall or with some other molecular species.
It happens that the first excited vibrational state of nitrogen, which lies at 2331 cm.sup.-1 above the ground vibrational state, coincides almost exactly in energy with the first excited state of the asymmetric stretching vibration of carbon dioxide at 2349 cm.sup.-1. Because of this near coincidence, excited nitrogen molecules can, upon collision with unexcited carbon dioxide molecules, efficiently transfer their energy to the carbon dioxide molecules, leaving the latter in the first excited state of its asymmetric stretch vibration, the upper state of its laser transitions. Since this excitation occurs preferentially, this first excited state of carbon dioxide becomes populated while the lower lying states remain unpopulated. As a result, a population inversion, much like that in a four-level laser, is immediately created with energy being released in the form of coherent radiation.
Although laser action will occur in carbon dioxide-nitrogen mixtures without the addition of other gases, the addition of helium or some other noble gas to the mixture increases the efficiency markedly. The helium or other noble gas acts to slow down the rate of energy loss from excited nitrogen atoms by wall collisions, moderates the energy of the discharge electrons, increases the nitrogen excitation efficiency, and spreads the discharge more uniformly throughout the active medium. Efficiencies of 30% have been observed in electric discharge pumped carbon dioxide-nitrogen-helium lasers, the maximum theoretical efficiency being given by the ratios of the energies of the laser state and of the laser quantum and is 41% for the 10.6 micron transition.
In copending application Ser. No. 680,252, filed Apr. 26, 1976, there is described a gas layer system which operates under the same basic principles as the carbon dioxide-nitrogen laser but which employs water vapor as the main lasing medium and hydrogen deuteride as the exciting molecule for the water vapor molecules in much the same manner as nitrogen molecules are used to excite carbon dioxide molecules as described above. While not mentioned in the foregoing copending application, hydrogen deuteride can be replaced by hydrogen with a small decrease in efficiency. It has been found, however, that in contrast to the carbon dioxide-nitrogen laser, the addition of a noble gas such as helium to the mixture actually decreases the efficiency of the system rather than increasing it. At the same time, it has been found that a relatively unstable discharge is achieved when hydrogen or hydrogen deuteride is used solely with water vapor.